Proliferating cell nuclear antigen (PCNA) is a non-oncogenic protein essential for tumor cell growth and survival. Native PCNA is present in nucleoplasm as ring-shaped homotrimers. To be functional, PCNA must be linearized or monomerized for relocalization. Upon linearized and loaded onto DNA, PCNA serves as platforms for partner proteins involved in DNA replication and repair, as well as other cellular processes. PCNA is also monomerized and relocalized to cytoplasm regulating glycolysis and apoptosis, and to cell membrane interrupting tumor cell recognition by natural killer cells. The PI and collaborators have discovered a novel class of PCNA inhibitors (PCNA-Is) that bind at the interfaces of two monomers, stabilize PCNA trimer structure, attenuate relocalization, and cause functional depletion of PCNA in cells. All tumor cells, including those with multidrug-resistant (MDR) phenotype and those with mutant p53 protein, are susceptible to PCNA-Is at the nanomolar concentrations. Treatment with PCNA-I1 significantly retards tumor growth in mice without apparent toxic effects on the hosts. A structure-activity relationship (SAR) analysis on PCNA-Is led to the identification of PCNA-I1S, which is more potent in stabilizing PCNA trimers and inhibiting PCNA relocalization and tumor cell growth. It inhibits chromatin association and nucleus-to-cytoplasm relocalization of PCNA, reduces recruitment of PCNA to DNA repair foci and induces DNA damage in quiescent cells, and attenuates PCNA interaction with glycolytic enzymes and glycolysis, a preferred energy source of cancer stem cells. As such, the PI hypothesized that the functional depletion of PCNA by blocking its relocalization with this novel class of PCNA-Is provides an effective modality for prostate cancer therapy. This hypothesis will be tested in two specific aims in this innovative research. Studies in aim 1 will determine the pharmacokinetics (PK) and tolerability of PCNA-I1S. The PK and bioavailability will be determined in mice by intravenous, oral, and intraperitoneally administration of PCNA-I1S. Multi-dose maximum tolerated dose (MTD) will be assessed by intravenous administration of PCNA-I1S. The drug distribution of PCNA-I1S will be determined in tumor-bearing mice. The metabolic stability of PCNA-I1S will be determined in vitro using the liver microsome assay. Researches in aim 2 will determine the efficacy and pharmacodynamics (PD) of PCNA-I1S therapy in prostate cancer models in mice. The efficacy of PCNA-I1S therapy against tumor growth and progression will be determined in castration-resistant prostate cancer 22Rv1 tumors and patient-derived xenografts (PDXs) in nude mice and in the transgenic adenocarcinoma mouse prostate (TRAMP) in immune competent mice. The therapeutic effects of PCNA-I1S will be correlated with its effects on PCNA chromatin association, apoptosis, and cell growth in tumor lesions. Potential systemic toxicity of PCNA-I1S will be further assessed in tumor-bearing mice.